The present invention relates to a device for adjusting the amount of slack in a lower thread in a zigzag sewing machine, especially, a zigzag sewing machine which contains a horizontal rotary loop taker to which the lower thread is supplied from a left side of the leftmost needle dropping point thereof. The present device adjusts the lower thread biasing leftward on a rear side of a fabric under zigzag stitching.
The prior art will be discussed with reference to the attached drawings for convenience. The zigzag sewing machine is, as shown in FIG. 2, contains a horizontal rotary loop taker 2 provided with a basic line disposed in front of a needle 1 which moves vertically and laterally. When straight stitching is carried out especially at the needle dropping leftmost point L (called "left basic line"), so-called hitch stitching will often occur. This is why the lower thread 4 from a bobbin 3 is disordered and it comes to the right side of the needle when the former crosses with the latter. For moving such occasions, as shown in FIG. 2, the lower thread 4 is set in such a way that it is pulled through a lower thread supply 2a and biased to the left side with regard to the left basic line L.
Thus, hitch stitching is avoided if the straight stitching is operated at the left basic line L. In this case, assume that the needle dropping rightmost point (called "right basic line") is R, the largest amplitude of the needle 1 is W, the length between the lower thread supply 2a and the left basic line L is (l.sub.1), and the length between the lower thread supply 2a and the right basic line R is (l.sub.2). Since the lower thread supply 2a is near the left side with regard to the left basic line L, and when the zigzag stitching is undertaken at the largest width W, then the slack amount of the lower thread when stitching on the left basic line L is approximately l.sub.2 +W, while the slack amount of the lower thread when stitching on the right basic line R can be approximately l.sub.1 +W. As is seen, since the amount of the former is more than the amount of the latter therefore, the lower thread 4 at the left basic line L is, as shown in FIG. 4, easily pulled up by an upper thread 5. With respect to the zigzag stitching of the sewn fabric, since a knotting 6.sub.L at the left side is near the left basic line L, and a knotting 6.sub.R at the right side is near the center of a stitch, the width W.sub.2 between the right knotting 6.sub.R at the right basic line R is therefore larger than the width W.sub.1 between the left knotting 6.sub.L and the left basic line L. Subsequently, when the tension of the upper thread 5 is slightly high, then the stitches of the lower thread 4 on the left basic line L easily appear to spoil the zigzag stitches 6.